Space–Time Pulsed Eddy Current Array for NDT of Wellbore Casings Based on MIMO Technique

In this article, we present a space–time pulsed eddy current (PEC) array for nondestructive testing (NDT) of wellbore casings using a multiple-input multiple-output (MIMO) technique. On the basis of a multilayered cylindrical wellbore model, an MIMO-PEC model is developed according to the phase shift characteristics of the PEC response corresponding to different transmitting–receiving (TR) distances. To achieve a more significant number of TR channels than in a traditional receiving array to reduce the complexity of the TR structure, a space–time transmission scheme is proposed to recover multiple TR channels. Moreover, a specific space–time MIMO array structure is designed to maximize the number of independent TR channels. The effectiveness of the proposed method is verified by applying the proposed MIMO array to a borehole PEC system for NDT of wellbore casings. The simulation and experimental results demonstrate that using the array structure and space–time transmission scheme proposed in this article, an MIMO structure consisting of <inline-formula> <tex-math notation="LaTeX">${M}$ </tex-math></inline-formula> transmitters and <inline-formula> <tex-math notation="LaTeX">${N}$ </tex-math></inline-formula> receivers can be equivalent to a transceiver system with a single transmitter and <inline-formula> <tex-math notation="LaTeX">${M}\,\,\times \,\,{N}$ </tex-math></inline-formula> receivers. Thus, MIMO technique can be effectively applied to PEC-NDT systems and drastically reduce the cost and complexity of the PEC system.